Abstract:

A H-bridge circuit for supplying a load with a defined current comprises a
first transistor for coupling a first terminal of the load to a positive
potential of a power source; a second transistor for coupling the first
terminal of the load to a negative potential of the power source; a third
transistor for coupling a second terminal of the load to the positive
potential of the power source; and a fourth transistor for coupling the
second terminal of the load to the negative potential of the power
source. The circuit further comprises a current sensing circuitry for
sensing a current flowing through the load and for generating a voltage
signal representative of the current, and a control circuit for
individually controlling the operation of the first, second, third and
fourth transistors by respective first, second, third and fourth control
signals. The control circuit is configured for operating two transistors
of the first, second, third and fourth transistors in switching mode and
the remaining two transistors in linear mode. Each of the linear mode
transistors has a feedback circuit associated therewith, which is
operatively coupled to the control circuit and to the current sensing
circuitry. Each such feedback circuit comprises a resistive element
coupled between the gate/base of the associated linear mode transistor
and the power source. The feedback circuits comprise a feedback
transistor coupled between the gate/base of the associated linear mode
transistor and the current sensing circuitry, the base/gate of the
feedback transistor being connected to the control circuit for receiving
the control signal for the associated linear mode transistor. The
feedback transistor controls the voltage on the gate/base of the
associated linear mode transistor so that a difference between the
voltage signal of said current sensing circuitry and the respective
control signal from said control circuit is maintained at a constant
level

Claims:

1.-10. (canceled)

11. A H-bridge circuit for supplying a load with a defined current,
comprisinga first transistor for coupling a first terminal of said load
to a positive potential of a power source;a second transistor for
coupling said first terminal of said load to a negative potential of said
power source;a third transistor for coupling a second terminal of said
load to said positive potential of said power source;a fourth transistor
for coupling said second terminal of said load to said negative potential
of said power source;a current sensing circuitry for sensing a current
flowing through said load and for generating a voltage signal
representative of said current, and a control circuit for individually
controlling the operation of said first, second, third and fourth
transistors by respective first, second, third and fourth control
signals, said control circuit being configured for operating two
transistors of said first, second, third and fourth transistors in
switching mode and for operating the remaining two transistors in linear
mode;wherein each of said linear mode transistors has a feedback circuit
associated therewith, said feedback circuit being operatively coupled to
said control circuit and to said current sensing circuitry,and wherein
said feedback circuit comprises a resistive element coupled between the
gate/base of the associated linear mode transistor and the power source,
and a feedback transistor coupled between the gate/base of the associated
linear mode transistor and the current sensing circuitry, the base/gate
of said feedback transistor being connected to said control circuit for
receiving the control signal for the associated linear mode
transistor,wherein said feedback transistor controls the voltage on the
gate/base of the associated linear mode transistor in such a way that a
difference between the voltage signal of said current sensing circuitry
and the respective control signal from said control circuit is maintained
at a substantially constant level.

12. The H-bridge circuit as claimed in claim 11, wherein said feedback
transistor is coupled with its emitter/source to said current sensing
circuitry and wherein said control signal for the associated linear mode
transistor is substantially equal to the sum of a reference signal
representative of the current signal to be supplied by the associated
linear mode transistor and the base-emitter voltage drop resp. the
gate-source voltage drop of the feedback transistor.

13. The H-bridge circuit as claimed in claim 11, wherein said first,
second, third and fourth transistors are MOSFET transistors.

14. The H-bridge circuit as claimed in claim 11, wherein said current
sensing circuitry comprises a shunt resistor coupled between the first
and the third transistor and said positive potential of said power source
or between the second and the fourth transistor and said negative
potential of said power source, such that the current flowing though said
load produces a potential difference across said shunt resistor.

15. The H-bridge circuit as claimed in claim 11,wherein the feedback
circuit associated to the one linear mode transistor comprises a first
feedback transistor and the feedback circuit associated to the other
linear mode transistor comprises a second feedback transistor,wherein
said control circuit comprises individual control nodes connected to the
base/gate of the first and second feedback transistor, respectively, for
outputting the control signals for the linear mode transistors;
andwherein each of said first and second feedback transistors is coupled
between the gate/base of the associated linear mode transistor and the
current sensing circuitry, respectively.

16. The H-bridge circuit as claimed in claim 15, wherein said first and
second feedback transistors are bipolar transistors.

17. The H-bridge circuit as in claim 11,wherein the feedback circuit
associated to the one linear mode transistor and the feedback circuit
associated to the other linear mode transistor comprise a common feedback
transistor, andwherein said control circuit comprises a common control
node connected to the base/gate of the common feedback transistor, said
common control node outputting the control signals for the linear mode
transistors in a temporally interlaced manner.

18. The H-bridge circuit as claimed in claim 17, wherein said common
feedback transistor is bipolar transistor.

19. The H-bridge circuit as claimed in claim 17, wherein said control
circuit comprises an electronic switch to switch said common feedback
transistor alternatively to the one or the other of the associated linear
mode transistors.

20. The H-bridge circuit as claimed in claim 19, wherein said electronic
switch is controlled with one of the control signals for controlling the
transistors operating in switching mode.

21. The H-bridge circuit as claimed in claim 18, wherein said control
circuit comprises an electronic switch to switch said common feedback
transistor alternatively to the one or the other of the associated linear
mode transistors.

22. The H-bridge circuit as claimed in claim 20, wherein said electronic
switch is controlled with one of the control signals for controlling the
transistors operating in switching mode.

Description:

TECHNICAL FIELD

[0001]The present invention generally relates to a H-bhdge circuit for
driving electrical loads with a defined current and to a method for
operating such an H-Bridge circuit in current sourcing mode.

BACKGROUND

[0002]It is well known in the art to use H-bhdge circuits for supplying a
defined current to a load. One specific method to do so is to use a fully
linear H-bridge which has low efficiency. Additionally, the measurement
of the actual value of the current into the load and an overall feedback
loop to control the current into the load are required.

[0003]A possible implementation of a H-bridge operating in linear mode is
shown in FIG. 1. In this circuit diagram, the different reference
numerals denote the following elements:

[0004]1 a power supply voltage source,

[0005]2, 3 NPN transistors,

[0006]4 the load,

[0007]5, 6 PNP transistors,

[0008]7 a shunt resistor, 8 the circuit ground,

[0009]9 a control circuit,

[0010]10 the supply voltage node,

[0011]11 the control node for transistor 2,

[0012]12 the control node for transistor 5, 13 the feedback node of the
load current measurement,

[0013]14 the control node for transistor 6,

[0014]15 the control node for transistor 7,

[0015]16 the left output node of the H-bridge,

[0016]17 the right output node of the H-bridge.

[0017]Bipolar transistors are shown in this figure, but other transistor
types, like Mosfet transistors can also be used instead. The load is
shown in the circuit diagram as resistor, but other load types, such as
complex or non-linear loads may also be used instead.

[0018]Transistors 2 and 5, and 3 and 6 each constitute a voltage follower.
The control circuit 9 supplies control voltages to the H-bridge in such a
way that the transistor bias currents are set correctly, so that the
current waveform through the load 4 is substantially equal to the
required current waveform, and that the amplitude of the current through
the load is substantially equal to the required amplitude. The transistor
bias currents are established by the control circuit by setting the
differences between the voltages of control nodes 11 and 12, and 14 and
15 respectively to a constant level. The required current waveform is
applied to the load by the control circuit by setting the mean of the
voltages of the control nodes 11 and 12 to the required waveform, and by
setting the mean of the voltages of the control nodes 14 and 15 to the
inverted required waveform. In this manner, the right output node of the
H-bridge, 17, is always in opposite phase compared to the left output
node of the H-bridge, 16. The control circuit 9 also measures the actual
value of the current amplitude through the load 4 by measuring the
voltage amplitude on feedback node 13, which is the amplitude of the
voltage drop through the shunt resistor 7, and thereby substantially
equal to the amplitude of the current through the load 4 multiplied by
the resistance of shunt resistor 7. A control loop inside the control
circuit 9 sets the signal amplitudes on the control nodes 11, 12, 14, 15
in such a way that the required load current amplitude is substantially
equal to the desired value.

[0019]A large disadvantage of this circuit is that a large minimum voltage
drop is required across the transistors 2, 3, 5 and 6, more precisely
between supply voltage node 10 and left output node 16, between supply
voltage node 10 and right output node 17, between left output node 16 and
feedback node 13, and between right output node 17 and feedback node 13.
These voltage drops imply that an appreciable amount of power is
dissipated in the transistors, which lowers the overall efficiency of the
H-bridge circuit.

[0020]Another disadvantage of this circuit is that the load current
amplitude must be controlled via an overall control loop, which can be
complex, and stability problems may arise depending on the load
characteristics.

[0021]Documents U.S. 2005/218843 and U.S. 2005/226018 disclose an H-bridge
circuit for applying a driving current to a motor. The H-bhdge circuit
comprises two linear-mode transistors and two switching-mode transistors
controlled by a state control circuit. A feedback circuit is provided,
which selectively applies the difference between the detected driving
current and a command signal as an error signal to either the one or the
other of the linear-mode transistors in such a way as to control the
driving current to become proportional to the command signal.

BRIEF SUMMARY

[0022]The invention provides an H-bhdge circuit for supplying a precisely
defined current to an electrical load in a more efficient way.

[0023]In order to overcome the above-mentioned problem, the present
invention proposes a H-bhdge circuit for supplying a load with a defined
current, comprising a first transistor for coupling a first terminal of
said load to a positive potential of a power source; a second transistor
for coupling said first terminal of said load to a negative potential of
said power source; a third transistor for coupling a second terminal of
said load to said positive potential of said power source; and a fourth
transistor for coupling said second terminal of said load to said
negative potential of said power source. The circuit further comprises a
current sensing circuitry for sensing a current flowing through said load
and for generating a voltage signal representative of said current, and a
control circuit for individually controlling the operation of said first,
second, third and fourth transistors by respective first, second, third
and fourth control signals, said control circuit being configured for
operating two transistors of said first, second, third and fourth
transistors in switching mode and for operating the remaining two
transistors in linear mode. According to the invention, each of said
linear mode transistors has associated therewith a feedback circuit,
which is operatively coupled to the control circuit and to the current
sensing circuitry. Each such feedback circuit comprises a resistive
element coupled between the gate/base of the associated linear mode
transistor and the power source. The feedback circuits further comprise a
feedback transistor coupled between the gate/base of the associated
linear mode transistor and the current sensing circuitry, the base/gate
of the feedback transistor being connected to the control circuit for
receiving the control signal for the associated linear mode transistor.
The feedback transistor controls the voltage on the gate/base of the
associated linear mode transistor so that a difference between the
voltage signal of said current sensing circuitry and the respective
control signal from said control circuit is maintained at a constant
level.

[0024]In contrast to the above described state of the art H-bhdge
circuits, the present invention operates one half of the H-bridge in
switching mode and the other in linear mode. Furthermore, each of the
linear mode transistors is controlled by a local feedback loop, which is
much more efficient and less complex than the prior art overall control
loop for controlling the load current amplitude.

[0025]In a preferred embodiment, said feedback circuit comprises a
feedback transistor coupled between the gate/base of the associated
linear mode transistor and the current sensing circuitry and a resistive
element coupled between the gate/base of the associated linear mode
transistor and the power source. The base/gate of said feedback
transistor is connected to said control circuit for receiving the control
signal for the associated linear mode transistor.

[0026]The feedback transistor is preferably coupled with its
emitter/source to said current sensing circuitry and said control signal
for the associated linear mode transistor is substantially equal to the
sum of a reference signal representative of the current signal to be
supplied by the associated linear mode transistor and the base-emitter
voltage drop resp. the gate-source voltage drop of the feedback
transistor. The reference signal is e.g. the voltage signal that the
desired current supplied by the associated linear mode transistor
generates at the current sensing circuitry.

[0027]According to a preferred variant of the invention, the feedback
circuits associated to the one linear mode transistor comprises a first
feedback transistor and the feedback circuit associated to the other
linear mode transistor comprises a second, separate, feedback transistor.
In this case, each of the first and second feedback transistors is
coupled between the gate/base of the associated linear mode transistor
and the current sensing circuitry, respectively. Furthermore, the control
circuit comprises individual control nodes connected to the base/gate of
the first and second feedback transistor, respectively.

[0028]It shall be noted that the feedback circuits can have components in
common: for instance, the feedback circuits could share a common feedback
transistor instead of two separate ones. According to another preferred
variant of the invention, the feedback circuit associated to the one
linear mode transistor and the feedback circuit associated to the other
linear mode transistor thus comprise a common feedback transistor. In
this variant, the control circuit comprises a common control node
connected to the base/gate of the common feedback transistor, which
outputs the control signals for both linear mode transistors in a
temporally interlaced (i.e. temporally multiplexed) manner. The control
circuit preferably comprises an electronic switch in order to switch the
common feedback transistor alternatively to the one and the other of the
associated linear mode transistors. Such electronic switch may e.g. be
controlled with one of the control signals for controlling the
transistors operating in switching mode.

[0029]The (first, second or common) feedback transistors are preferably
bipolar transistors, which are characterized by a more stable operation
under varying temperature conditions. On the other hand, the first,
second, third and fourth transistors are preferably MOSFET transistors,
which are known for their faster switching and their reduced power
consumption.

[0030]In a very simple embodiment, the current sensing circuitry comprises
a shunt resistor coupled between the first and the third transistor and
said positive potential of said power source or between the second and
the fourth transistor and said negative potential of said power source,
such that the current flowing though said load produces a potential
difference across said shunt resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]Preferred embodiments of the invention will now be described, by way
of example, with reference to the accompanying drawings in which:

[0032]FIG. 1 shows a circuit diagram of a prior art H-bhdge circuit; FIG.
2 shows a circuit diagram of a first embodiment of a H-bridge circuit
according to the present invention;

[0033]FIG. 3 illustrates the different signals during the operation of the
circuit shown in FIG. 2;

[0034]FIG. 4 shows a circuit diagram of a second embodiment of a H-bridge
circuit according to the present invention;

[0035]FIG. 5 illustrates the different signals during the operation of the
circuit shown in FIG. 4.

DETAILED DESCRIPTION

[0036]A possible implementation of a H-bridge in accordance with the
present invention is shown in FIG. 2. In this circuit diagram, the
different reference numerals denote the following elements:

[0037]1 a power supply voltage source,

[0038]4 the load,

[0039]7 a shunt resistor, 8 the circuit ground,

[0040]10 the supply voltage node,

[0041]20 a control circuit,

[0042]21, 24 P-channel Mosfets,

[0043]22, 23 N-channel Mosfets, 25, 26 NPN transistors,

[0044]27, 28 pullup resistors,

[0045]31 the control node for transistor 25,

[0046]32 the control node for Mosfet 22,

[0047]33 the control node for Mosfet 23, 34 the control node for
transistor 26,

[0048]35 the upper node of shunt resistor 7.

[0049]The curves in FIG. 3 illustrate the different signals during the
operation of the circuit shown in FIG. 2.

[0050]40 an example of the desired current waveform,

[0051]41 the waveform of the voltage on control node 31, 42 the waveform
of the voltage on control node 32,

[0052]43 the waveform of the voltage on control node 33,

[0053]44 the waveform of the voltage on control node 34,

[0054]45 the waveform of the voltage on upper node 35 of shunt resistor 7,

[0055]46 the current through the load 4, 50 a voltage which is
substantially equal to the base-emitter voltage drop (Vbe) of transistors
25 and 26 (e.g. =0.7 V for bipolar transistors).

[0056]Four periods of the relevant signals in FIG. 2 are shown in FIG. 3.
The signals shown in FIG. 3 are only an illustrative example for signals
which may occur in an implementation of the circuit shown in FIG. 2. When
a different load current signal is required, the signal forms shown in
FIG. 3 will be different.

[0057]The control circuit 9 transforms the required signal 40 into the
four control signals 41, 42, 43, 44 on control nodes 31, 32, 33, 34
respectively. The control circuit needs not necessarily transform the
required signal into the control signals, it can also generate the
control signals individually from scratch.

[0058]Control signal 42 is high whenever the required signal 40 is
positive and low otherwise. Control signal 43 is high whenever the
required signal 40 is negative and low otherwise.

[0059]Control signal 44 is substantially equal to the sum of the required
signal 40 and the base-emitter voltage drop 50 (Vbe) of transistors 25
and 26 during the first half of each period, and substantially equal to
the base-emitter voltage drop 50 (Vbe) of transistors 25 and 26 during
the second half of each period.

[0060]Control signal 41 is substantially equal to the sum of the required
signal 40 and the base-emitter voltage drop 50 (Vbe) of transistors 25
and 26 during the second half of each of its periods, and substantially
equal to the base- emitter voltage drop 50 (Vbe) of transistors 25 and 26
during the first half of each of its periods.

[0061]In the circuit of FIG. 2, transistors 21 and 24 are operated as
controlled current sources, while transistors 22 and 23 are operated as
switches with low switch voltage drop.

[0062]During the first half period of the required signal 40, the control
signal 42 is set to high level, thereby switching on transistor 22.
Control signal 43 is low, thereby switching off transistor 23. Control
signal 44 is substantially equal to the sum of the required signal 40 and
the base-emitter voltage drop 50 (Vbe) of transistor 26. Control signal
41 is substantially equal to the base-emitter voltage drop 50 (Vbe) of
transistor 26. Transistors 24, 22 and 26, load 4 and shunt resistor 7
form a first load current control loop during the first half of each
period. The supply current flows from the power supply voltage source 1
through transistor 24, the load 4, transistor 22, shunt resistor 7, into
circuit ground 8 and back into supply voltage source 1. The
aforementioned first load current control loop keeps the current through
the load substantially equal to the required signal 40 divided by the
resistance of the shunt resistor 7 in the following way: the current
through the load also flows through the shunt resistor 7, thereby
creating a voltage on the upper node 35 of the shunt resistor 7, which is
equal to the current through the load multiplied by the resistance of
shunt resistance 7.The difference between the voltage on the control node
34 and the voltage on the node 35 is equal to the voltage at the
base-emitter input voltage of transistor 26. The base-emitter voltage
sets, by the well-known exponential relationship between base-emitter
voltage and collector current of a bipolar transistor, the collector
current of transistor 26. If the aforementioned difference between the
voltage on the control node 34 and the voltage on the node 35 increases,
the collector current into transistor 26 increases as an exponential
function of the aforementioned voltage difference. This collector current
flows through the pullup resistor 28, thereby pulling down the voltage on
the gate of transistor 24, and thereby increasing the current flowing
through transistor 24 (because, in FIG. 2, transistor 24 is a P-channel
MOSFET connected in common source configuration). This same current flows
from the supply voltage node 10 into the load 4, thereby increasing the
current through shunt resistor 7, and thereby increasing the voltage on
the upper node 35 of the shunt resistor 7. Essentially the same procedure
happens, but in opposite direction, if the aforementioned difference
between the voltage on the control node 34 and the voltage on the node 35
decreases.

[0063]As the relation between the base-emitter voltage and the collector
current is exponential, and the collector current will settle to a value
which is determined by resistor 28 and the threshold gate voltage of
transistor 24, whereby this value will typically not vary more than one
order of magnitude, the base-emitter voltage, and thus the difference
between the voltage on the control node 34 and the voltage on the node 35
will remain substantially constant due to the aforementioned exponential
relationship, typically around 0.6V to 0.7V. A typical value for the
resistance of resistor 28 is 1 k Ohm for example and a typical value for
the threshold gate voltage of transistor 24 is for example 5V for a load
current of 1 A. In summary, the first control loop adjusts the current
through the load until the voltage on the upper node 35 of the shunt
resistor 7, signal 45 in FIG. 3, is substantially equal to the voltage on
the control node 34 minus the base-emitter voltage drop 50 (Vbe) of
transistor 26, which is substantially equal to the required signal 40.

[0064]During the second half period of the required signal 40, the control
signal 43 is set to high level, thereby switching on transistor 23.
Control signal 42 is low, thereby switching off transistor 22. Control
signal 41 is substantially equal to the sum of the required signal 40 and
the base-emitter voltage drop 50 (Vbe) of transistor 25. Control signal
44 is substantially equal to the base-emitter voltage drop 50 (Vbe) of
transistor 25. Transistors 21, 23 and 25, load 4 and shunt resistor 7
form a second load current control loop during the second half of each
period. The supply current flows from the power supply voltage source 1
through transistor 21, the load 4, transistor 23, shunt resistor 7, into
circuit ground 8 and back into supply voltage source 1. Similarly to the
first control loop, the second control loop controls the current through
the load, signal 46 in FIG. 3, in such a way that it is substantially
equal to the required signal divided by the resistance of shunt resistor
7 during the second half of each period

[0065]Finally, as the two control loops operate alternatively, the current
through the load 4 is consequently always substantially equal to the
required signal divided by the resistance of shunt resistor 7.

[0066]It is obvious that the control signal 44 needs not to be kept
substantially equal to the base-emitter voltage drop 50 (Vbe) of
transistor 26 during the first half of each period, it is sufficient that
the voltage level of control signal 44 is sufficiently low so that
transistor 26 is kept off during the first half of each period. Also, it
is obvious that the control signal 41 needs not to be kept substantially
equal to the base-emitter voltage drop 50 (Vbe) of transistor 25 during
the first half of each period, it is sufficient that the voltage level of
control signal 41 is sufficiently low so that transistor 25 is kept off
during the first half of each period.

[0067]The required voltage level shift of one base-emitter voltage drop
(Vbe) of transistors 25 and 26, between the required signal and the
control signal 44 during the first half of each period, and between the
required signal and the control signal 41 during the second half of each
period can easily be implemented by using one additional transistor for
each of the level shifts, for example a PNP bipolar transistor configured
as voltage follower.

[0068]It will be noted, that instead of the bipolar transistors shown in
FIG. 2 other transistor types, like Mosfet transistors can be used
instead, and instead of the Mosfet transistors shown in this figure other
transistor types, like bipolar transistors can be used instead.
Furthermore the load is shown in the circuit diagram as resistor, but
other load types, such as complex or non-linear loads may also be used
instead.

[0069]The pullup resistors 27 and 28 can also be replaced by current
sources.

[0070]The control circuit 20 can be any electronic circuit that is capable
of generating the desired signals of nodes 31, 32, 33, 34. In a typical
application, control circuit 20 can be a microcontroller with digital
output signal nodes 32, 33 and with two additional digital-to-analog
converters connected to and driven by the microcontroller, where the
outputs of the digital-to-analog converters then correspond to signal
nodes 31 and 34 in FIG. 2. In another typical implementation, control
circuit 20 can be a field programmable array (FPGA) with digital output
signal nodes 32, 33 and with two additional digital-to-analog converters
connected to and driven by the FPGA, where the outputs of the
digital-to-analog converters then correspond to signal nodes 31 and 34 in
FIG. 2.

[0071]Another possible implementation of a H-bridge in accordance with the
present invention is now discussed with reference to FIGS. 4 and 5. In
this implementation, the feedback circuits share a common feedback
transistor, which alternately controls the linear mode transistors. In
the circuit diagram of FIG. 4, the different reference numerals denote
the following elements:

[0072]101 a power supply voltage source,

[0073]104 the load, 107 a shunt resistor,

[0074]108 the circuit ground,

[0075]110 the supply voltage node,

[0076]120 a control circuit,

[0077]121, 124 P-channel Mosfets, 122, 123 N-channel Mosfets,

[0078]125 NPN transistor,

[0079]127, 128 are pullup resistors,

[0080]132 the control node for Mosfet 122,

[0081]133 the control node for Mosfet 123 and electronic switch 138, 135
the upper node of shunt resistor 107,

[0082]138 an electronic switch,

[0083]139 the control node for transistor 125.

[0084]Reference numeral 138 designates an analog electronic switch. The
switching state is controlled by the control input connected to node 133.
When the control input is set to a low level, the switch 138 is set to a
first state in which the collector of transistor 125 is connected to the
gate of transistor 124 and the gate of transistor 121 is not connected to
the collector of transistor 125. When the control input is set to a high
level, the switch 138 is set to a second state in which the collector of
transistor 125 is connected to the gate of transistor 121 and the gate of
transistor 124 is not connected to the collector of transistor 125. An
example for electronic switch 138 is the integrated circuit CD4053 from
Fairchild Semiconductor but there are numerous alternatives available on
the market.

[0085]The curves in FIG. 5 illustrate the different signals during the
operation of the circuit shown in FIG. 4. 140 an example of the desired
current waveform,

[0086]142 the waveform of the voltage on control node 132,

[0087]143 the waveform of the voltage on control node 133,

[0088]145 the waveform of the voltage on upper node 135 of shunt resistor
107,

[0089]146 the current through the load 104,

[0090]149 the waveform of the voltage on control node 139,

[0091]150 a voltage that is substantially equal to the base-emitter
voltage drop (Vbe) of transistor 125 (e.g. =0.7 V for bipolar
transistors).

[0092]For enabling easier comparison with the embodiment of FIGS. 2 and 3,
FIG. 5 also shows the waveforms 41 and 44 for the control nodes 31 and 34
of FIG. 2.

[0093]Four periods of the relevant signals in FIG. 4 are shown in FIG. 5.
The signals shown in FIG. 5 are only an illustrative example for signals
that may occur in an implementation of the circuit shown in FIG. 4. When
a different load current signal is required, the signal forms shown in
FIG. 5 will be different.

[0094]The control circuit 109 transforms the required signal 140 into the
control signals 142, 143, 149 on control nodes 132, 133, 139
respectively. The control circuit needs not necessarily transform the
required signal into the control signals; it can also generate the
control signals individually from scratch.

[0095]Control signal 142 is high whenever the required signal 140 is
positive and low otherwise. Control signal 143 is high whenever the
required signal 140 is negative and low otherwise.

[0096]The control signals for the feedback circuit associated with linear
mode transistor 121 and for the feedback circuit associated with linear
mode transistor 124 are time-multiplexed in signal 149. During the first
half of each period of the required signal 140, signal 149 is
substantially equal to the sum of the required signal 140 and the
base-emitter voltage drop 150 (Vbe) of transistor 125 (and thus
correspond to control signal 44 during these time intervals). During the
second half of each period of the required signal 140, signal 149 is
substantially equal to the sum of the inverted required signal 140 and
the base-emitter voltage drop 150 (Vbe) of transistor 125 (and thus
correspond to control signal 41 during these time intervals).

[0097]In the circuit of FIG. 4, transistors 121 and 124 are operated as
controlled current sources, while transistors 122 and 123 are operated as
switches with low switch voltage drop.

[0098]During the first half period of the required signal 140, the control
signal 142 is set to high level, thereby switching on transistor 122.
Control signal 143 is low, thereby switching off transistor 123 and
connecting the collector of transistor 125 to the gate of transistor 124
through the action of the electronic switch 138. Control signal 149 is
substantially equal to the sum of the required signal 140 and the
base-emitter voltage drop 150 (Vbe) of transistor 125.

[0099]Transistors 124, load 104, transistor 122, shunt resistor 107,
transistor 125 and electronic switch 138 form a load current control loop
during the first half of each period. The supply current flows from the
power supply voltage source 101 through transistor 124, the load 104,
transistor 122, shunt resistor 107, into circuit ground 108 and back into
supply voltage source 101. The aforementioned load current control loop
keeps the current through the load substantially equal to the required
signal 140 in the following way: the current through the load flows
through the shunt resistor 107, thereby creating a voltage on the upper
node 135 of the shunt resistor 107, which is equal to the current through
the load multiplied by the resistance of shunt resistance 107.

[0100]The difference between the voltage on the control node 139 and the
voltage on the node 135 is equal to the voltage at the base-emitter input
voltage of transistor 125. The base-emitter voltage sets, by the
well-known exponential relationship between base-emitter voltage and
collector current of a bipolar transistor, the collector current of
transistor 125. If the aforementioned difference between the voltage on
the control node 139 and the voltage on the node 135 increases, the
collector current into transistor 125 increases as an exponential
function of the aforementioned voltage difference. This collector current
flows through the electronic switch 138 and the pullup resistor 128,
thereby pulling down the voltage on the gate of transistor 124, and
thereby increasing the current flowing through transistor 124 (because,
in FIG. 4, transistor 124 is a P-channel MOSFET connected in common
source configuration). This same current flows from the supply voltage
node 110 into the load 104, thereby increasing the current through shunt
resistor 107, and thereby increasing the voltage on the upper node 135 of
the shunt resistor 107. Essentially the same procedure happens, but in
opposite direction, if the aforementioned difference between the voltage
on the control node 139 and the voltage on the node 135 decreases.

[0101]As the relation between the base-emitter voltage and the collector
current is exponential, and the collector current will settle to a value
which is determined by resistor 128 and the threshold gate voltage of
transistor 124, whereby this value will typically not vary more than one
order of magnitude, the base-emitter voltage, and thereby the difference
between the voltage on the control node 139 and the voltage on the node
135 will remain substantially constant due to the aforementioned
exponential relationship, typically around 0.6V to 0.7V. A typical value
for the resistance of resistor 128 is 1 k Ohm for example and a typical
value for the threshold gate voltage of transistor 124 is for example 5V
for a load current of 1 A. In summary, the control loop increases or
decreases the current through the load until the voltage on the upper
node 139 of the shunt resistor 107, signal 145 in FIG. 5, is
substantially equal to the voltage on the control node 139 minus the
base-emitter voltage drop 150 (Vbe) of transistor 125, which is
substantially equal to the required signal 140.

[0102]During the second half period of the required signal 140, the
control signal 143 is set to high level, thereby switching on transistor
123 and connecting the collector of transistor 125 to the gate of
transistor 121 through the action of the electronic switch 138. Control
signal 142 is low, thereby switching off transistor 122. Control signal
149 is substantially equal to the sum of the inverted required signal 140
and the base-emitter voltage drop 150 (Vbe) of transistor 125.

[0103]Transistors 121, load 104, transistor 123, shunt resistor 107,
transistor 125 and electronic switch 138 form a load current control loop
during the second half of each period. The supply current flows from the
power supply voltage source 101 through transistor 121, the load 104,
transistor 123, shunt resistor 107, into circuit ground 108 and back into
supply voltage source 101. Similarly to the control loop of the first
half of each period, the control loop now controls the current through
the load.

[0104]The control circuit 120 can be any electronic circuit that is
capable of generating the desired signals on nodes 132, 133, 139. In a
typical application, control circuit 120 can be a microcontroller with
digital output signal nodes 132, 133 and with a digital-to-analog
converters connected to and driven by the microcontroller, where the
output of the digital-to-analog converter then corresponds to signal node
139 in FIG. 4. In another typical implementation, control circuit 120 can
be a field programmable array (FPGA) with digital output signal nodes
132, 133 and a digital-to-analog converter connected to and driven by the
FPGA, where the output of the digital-to-analog converter then
corresponds to signal node 139 in FIG. 4.

[0105]It should be noted that the desired current waveforms 40 and 140
represented in FIGS. 3 and 5, respectively, needs not to be physically
present in the circuit. If the signal is present, the control circuits 20
and 120 can use it as a template to generate the control signals.